The present invention relates to automatic machines for infusion making, in particular for making espresso coffee, and more particularly it concerns an infusion group for one such machine.
Some kinds of infusion making machines include an infusion group comprising a chamber intended to receive the powdered substance, and a pair of confronting pistons which penetrate into the chamber and between which the powder is compressed. Suitable ducts supply hot water for preparing the infusion so that water passes through the powder, and collect and convey outside the machine the infusion. The main problems encountered in such machines are the compression control and the disposal of the used substance at the end of the operations, so as to leave the infusion group clean and ready for making a new infusion dose.
An example of infusion group of that type is disclosed in U.S. Pat. No. 4,852,472. In that infusion group the chamber intended to receive the ground coffee is formed by the internal cavity of an axially sliding cylinder. The cylinder is associated with a first moving piston mounted so as to move together with the cylinder when the latter is translated to compress the coffee, and to remain in that position, to eject the used coffee dose, during part of the return stroke of the cylinder. The second piston can slide for a short length against the action of a spring because of the compression exerted by the first piston. Filter cleaning is ensured by a water jet introduced in similar manner to the infusion water.
That infusion group is simple, since the motion of a single part is to be controlled, and allows an easy disposal of the used coffee and an easy filter cleaning; yet the control of the powder compression degree, and hence of the infusion quality, is difficult because of the relative movement of the two pistons.
Another infusion group including a chamber movable relative to a pair of confronting pistons is disclosed in BE-A 745 939. In the infusion group disclosed in that document, the infusion chamber is formed in a cylinder with horizontal axis, which is made to slide horizontally relative to a first stationary piston by a pair of threaded shafts rotating in the same direction. The first piston is housed within the cylinder cavity and divides it, when the infusion is being prepared, into two chambers containing the coffee and additional ingredients, such as milk and/or sugar, respectively. The second piston is a moving piston and it is horizontally displaced, when the cylinder has reached an end-of-stroke position, by the same shaft pair as that driving the cylinder, which shafts continue rotating in the same direction. The displacement of the second piston brings it inside the cylinder cavity to compress the ground coffee against the first piston. Infusion water is fed through the first piston and, after passing through the coffee, enters into the chamber containing the additional ingredients, where the coffee infusion is mixed with said ingredients by means of a mixer. Ejection of the used coffee can take place by gravity or can be caused by a blade used to clean the filters.
Coffee compression between a piston driven by own driving means and a fixed piston allows a good control of the compression degree. Yet the prior art device has a great structural complexity, due to the driving of the cylinder and the moving piston in subsequent steps, by means of the same shaft pair, and to the need of a suitable device for cleaning the filters.
The invention aims to provide an infusion group in which a good control of the powder compression is ensured by means of a very simple structure.
The infusion group according to the invention comprises a moving member with an internal cavity open at both ends and adapted to receive a dose of powdered substance supplied through introduction means, and means for causing an axial translation of said member relative to a pair of confronting pistons, of which one is a moving piston connected to said translation means so as to be axially displaced, and the other is a stationary piston, said pistons being adapted to co-operate with said cavity by tightly engaging the cavity walls in order to define an infusion chamber, and having heads equipped with filters between which said dose of powdered substance can be compressed because of the translation movements of said member and said moving piston. The groups is characterised in that said translation means comprise a first and a second threaded shaft that are associated with said moving member and said moving piston, respectively, and are simultaneously rotated in opposite directions by a driving member so as to cause simultaneous axial translation, in opposite directions, of the moving member and the moving piston.